Generally, video display terminals, television cabinets, and other monitoring devices, hereinafter referred to as video display units, include a video display device, such as a cathode ray tube (CRT), and a preformed bezel. Often these video display units also have a housing or some other structural support which is fastened to the bezel.
In order to fasten a preformed video display screen bezel securely to a CRT, at least four factors should be considered. These include the normal dimensional variations of CRTs, pressure related stresses exerted by the bezel fastening apparatus on the CRT, the size of the bezel and CRT, and the cost of the video display unit. Often damaging pressure-induced stresses can be exerted upon the CRT when attempting to firmly attach the bezel to the CRT due to dimensional variations in the CRT. Such stresses can cause the CRT to crack or implode. To avoid these problems, extra effort is required, which can increase the size and cost of the video display units significantly.
As a practical matter, it is difficult to cast or mold CRTs to exact dimensions, for as the glass envelope of the CRT cools it will shrink unpredictably. The display screen surface is brought into strict tolerance by grinding and polishing or by a separate, polished, tightly toleranced glass screen surface fixed onto the existing screen. However, the other elements of the CRT may not be brought easily within strict tolerances. For instance, the circumference of a 19 inch CRT can vary by as much as .+-.78 mils. The fastening apparatus used to connect the bezel to the CRT must accommodate this variation to fit securely.
The two most common bezel fastening apparatus are mechanical fasteners and elastomeric adhesive materials. If the bezel is mounted mechanically, care must be taken not to concentrate undue pressure on the CRT at the mounting points. If the bezel is mounted with an adhesive material, care must be taken to avoid pressure-induced stresses on the CRT which may be created by the adhesive material as it cures or as the adhesive material changes dimensions due to changes in environmental variables such as temperature or humidity.
The use of mechanical fasteners to secure the bezel to the CRT may induce stress due to a concentration of pressure on small areas of the CRT. This stress can lead to the CRT imploding or cracking.
Often, mechanical fasteners compensate for the dimensional variations of CRTs by incorporating expanded alignment margins, which may increase the size of the fasteners. The bezel is required to surround and conceal these mechanical devices and fasteners. Thus, as the fastener size increases, the bezel size must also increase.
Furthermore, once assembled, mechanical fasteners frequently leave the CRT prone to highly undesirable shifting within the bezel which may occur under accidental dynamic loads experienced in shipping or under shock and vibration. The degree to which the mechanical fasteners can be tightened is limited due to possible damage to the CRT or the fasteners themselves.
Mechanical fasteners, whether many simple ones or few complicated ones require a substantial amount of time for assembly which adds to the manufacturing cost. In addition, most mechanical fasteners are difficult to apply automatically, and therefore, their use increases assembly time.
When using an adhesive to fasten a preformed bezel to the CRT, an elastomeric adhesive material, such as silicon resin, may be inserted into a recess between the bezel and the CRT. The fluid state of the adhesive material allows it to conform to the CRT and to compensate for any dimensional variations of the CRT. The bezel is typically designed to be large enough to accommodate the maximum expected dimensional variations of the CRT, while still allowing it to be attached to the CRT by the elastomeric adhesive material.
The assembly of such video display units may involve a lengthy process. The CRT first needs to be positioned within the preformed bezel in such a way as to center the display screen with respect to the front opening in the bezel. Once the positioning is correct, the CRT is held in place while the adhesive material is inserted between the CRT and the bezel and until the adhesive material has cured. The cure time for elastic materials may be considerable, for example, from several hours to overnight, resulting in a long assembly time and increased manufacturing cost.
Stresses may be exerted on the CRT by the adhesive material as it cures. Typical adhesives may be thermoplastic which become plastic and flow upon heating so that they can be inserted into the recess between the preformed bezel and the CRT. As the thermoplastic material cures, it cools and contracts. This contraction can exert pressure on the CRT producing corresponding stresses.
Following manufacturing, the video display unit may also be exposed to wide temperature variations including severe cold during shipping and storage and relatively high heat when operated in environments with no temperature control. These temperature variations cause the adhesive material to contract and expand, and thereby change dimension, which exerts pressure-related stresses on the CRT. When it contracts, the adhesive directly squeezes the CRT. When it tries to expand, the bezel may confine it, again causing it to exert force on the CRT.
One way to compensate for the expansion of the adhesive material is to provide a void between the preformed bezel and the CRT in the rearward portion of the video display unit. The adhesive material may expand into this void when the device is operated in a heated environment. This attempts to relieve any undesirable axial shifting of the CRT within the preformed bezel due to forces exerted on the frustro-conical section, typical of most CRTs, by the thermally expanding elastic material. This method does not address the pressure related stresses exerted on the CRT as the elastic material shrinks.
It is therefore desirable to provide a low cost video display unit which avoids the foregoing and other disadvantages of known units, and it is to these ends the present invention is directed. The invention eliminates the possibility of the CRT shifting within the bezel due to shock and vibration, reduces stress induced by the bezel fastening apparatus on the CRT, and compensates for the dimensional variations of the CRT while maintaining minimum overall video display unit dimensions, and affords other benefits and advantages.